1. Field of the Invention
This invention relates to information signal transmitting apparatuses, and more particularly, to a noise reduction circuit for enlarging the apparent dynamic range of a signal transmission system or a recording and reproducing apparatus by alternately compressing and expanding an information signal.
2. Description of the Prior Art
Noise reduction circuits generally operate by compressing a signal at the time of recording or transmission and by expanding the compressed signal at the time of reception or playback whereby the apparent dynamic range of the recording or transmission medium is expanded. A compression circuit is accordingly provided at the signal input point, and an expansion circuit is provided at the signal output point. The compression and expansion circuits include variable transfer function circuits which adjust the gain of signals supplied thereto. Control circuits are also included to change the transfer function in response to the signal level and the frequency of the signal. Transfer function circuits and control circuits, however, have predetermined response time constants and occasionally exhibit undesirable transfer responses when an input signal supplied thereto changes very suddenly by a large amount.
In a compression circuit, for example, the gain is increased to a very large value when the input signal level is low, and increased to a smaller value when the input signal level is high. When an input signal level increases suddenly, however, a high level signal is applied to a circuit which has a transiently high gain state due to the response time constant, so that an output signal from the circuit has an extremely large magnitude or overshoot. It is to be appreciated that a recording medium has a maximum allowable level, known as a saturation level, so that when an input signal exceeds the maximum allowable level, the output signal from the expansion circuit is not a precise copy or replica of the original input signal. Prior art noise reduction circuits thus include amplitude limiting circuits in the compression circuit so that the signal amplitude is lower than the maximum allowable amount to prevent an overshoot condition.
Amplitude limiting circuits are usually formed of non-linear elements using semi-conductor PN junctions, such as diodes. A diode limiter is shown, for example, in U.S. Pat. No. 3,631,365.
Non-linear elements such as diodes have limited applicability, however, because of their fixed limiter levels. For example, a silicon PN junction has a forward threshold level approximately equal to 0.6 V. An amplitude limiting circuit including a reversed parallel circuit of two silicon diodes (in which the anodes and the cathodes are connected in reverse order) has a limiter level approximately equal to 1.2 V.sub.p-p (peak to peak value). The limiter level tends to be considerably larger than the signal level in the neighborhood of a junction in the amplitude limiting circuit. Hence, the limiter level is impractically large when the silicon diode limiter is used unmodified in the compression circuit.
According to the prior art, an amplifier is connected to the input side of the silicon diode limiter and sets an adequate limiter level relative to the signal level in the neighboring circuit portions by preamplifying the signal supplied to the limiter. It is also necessary to damp the limiter output signal to the same extent that the input signal is amplified in order to restore the output signal to its original level. Thus, an amplifier and a damper are connected to the input and output terminals, respectively, of the limiting circuit to adjust the limiter level to a desired optimum level between the input signal and the output signal of the circuit comprising the amplifier, the limiter and damper, regardless of the limiter levels of the elements.
However, such an arrangement suffers from significant drawbacks. First, an amplifier is required to set an actual limiter level which differs from the limiter level of the limiter elements, thereby complicating the circuit. Second, an extra damping operation must be performed on the signal, thus causing a deterioration in the signal to noise ratio of the circuit and reducing the precision of the compression operation. In addition, when a diode limiter is connected to a compression circuit, it is generally connected between the signal channel and ground. Distortion current can flow through the limiter and affect other circuit portions through a common impedance of the grounded circuit.
When a limiter circuit is included in a recording and/or reproducing apparatus such as a tape recorder, the limiter levels must be selected between two or more magnitudes since the recording characteristics of magnetic tape vary from tape to tape. Also, the limiter level must be changed when one noise reduction circuit is changed for another. Accordingly, a switch circuit must be provided to change the limiter values of the limiter circuit.